1. Field of the Invention
The present invention relates to calibration devices, methods, and computer-readable recording medium for storing calibration programs, which, in an image input/output process, obtain and store a desired input/output characteristic.
2. Description of Art
Conventionally, an image forming device, such as a printer, generates a predetermined image signal through an image process based on print data that is inputted, and adjusts the toner density based on the image signal. This results in a printed image having proper gray levels. In this type of forming device, in order to make the image signal coincide with the toner density actually printed, a correction of a so-called gamma characteristic (density/gray level characteristic) is generally performed.
FIGS. 15A to 15C illustrate a principle of the gamma correction process. As shown in FIG. 15C, although the gamma characteristic appears ideal when configured such that a change in the output is linear with respect to a change in the input when perceived by a person (the output changes with respect to the input as a designer intended), in reality, the gamma characteristic varies depending on the respective devices. Accordingly, it is therefore difficult to provide an ideal gamma characteristic.
In order to address this problem, in the image forming device, a gamma correction table or curve is provided (see FIG. 15B). This table corrects the unique gamma characteristic of the device at an initial setting when the device is manufactured, see FIG. 15A, thereby providing the ideal gamma characteristic.
When the gamma characteristic of the image forming device and the desired gamma characteristic are provided, the gamma correction table is obtained by an inverse operation from those gamma characteristics.
However, due to a change over time caused by long term operation or the like, the density of the toner may deviate from the gamma characteristic that is initially set. Therefore, it is necessary in conventional image forming devices, to correct the gamma characteristic periodically, or over a pre-set time period where the deviation of the gamma characteristic exceeds a threshold, so the actual toner density is properly set.
Typically, a calibration process is used, wherein the toner is attached to the photosensitive member according to some gray levels, and the densities of the attached toner are measured by a sensor. The gamma correction table is then calibrated based on the measurements obtained from the sensor so that the actual gamma characteristic approaches the ideal gamma characteristic.
For example, when an actual maximum density value which the device can output exceeds a predetermined maximum density value, a calibration is performed causing the density value to smoothly and monotonically increase to the actual maximum density value. On the other hand, when the actual maximum density value that can be output by the device does not reach the predetermined maximum density value, a calibration is performed so that the maximum density value is in a range from an input value which first causes the device to output the maximum density value to the maximum input value.